Dr. Harry said:On an different line of thought, I was considering how complicated reasonable copyright laws are. For a single work, setting some period of time - let me just grab the random number of (roll, roll, roll) ten years - seems reasonable, but there are a number of cases where this seems unjust. Consider an ongoing work; the Harry Potter series, or the Batman comic, or the Peanuts strip. By having copyright expire in these cases, then J.K. Rowling loses the income from the first book in the series about the time the last one will come out; anybody can put out a Batman comic, and Charles Schulz would only have been in control of a tiny fraction of his work.
It would seem reasonable to differentiate between the ability to copy material and the ability to use the characters, though I can see some limitation based on a requirement to keep in characters in play to keep the rights protected.
Don't forget trademark. While the old Batman comics might become PD, Batman himself wouldn't, so others still couldn't write new Batman stories. I'm not sure i like this state of affairs, but if it didn't also change, shortened copyright would do essentially nothing to threaten the holders of valuable entertainment IP, almost all of which has distinctive trademarkable elements. Disney is in no danger of losing exclusive control over Mickey Mouse (or any other character) just because Steamboat Willie falls into the public domain.
And, in any case, copyright renewals could very well take care of that. They should not be cheap--while initial copyright must be free to do its job, renewals could be expensive enough to discourage their use except for things like very valuable series. If i were J.K. Rowling, i'd gladly pay a few 10s of thousands of dollars to renew the copyright on the first couple of Harry Potter books until all of the series was out. It might even be possible to automatically extend copyright in some manner where series are concerned, though it'd take some careful rules to balance the benefits with the potential for abuse.